This invention relates to a doctor blade apparatus, having a doctor blade fastened in a doctor blade holder to be positioned along a cylinder capable of taking up paint, or ink, for example a form cylinder of a printing machine for intaglio engraving.
With doctor blade apparatus of this kind there exists the problem of positioning the doctor blade with constant pressure over its whole length against the cylinder, even if the surface line of the cylinder which abuts the doctor blade, is not exactly parallel to the cutting edge of the blade, which is e.g. the case when the axis of the cylinder is not parallel to the edge of the blade or if the cylinder has a conical shape.
In a doctor blade apparatus which is disclosed in DE-AS 12 73 540, good pressure of the doctor blade against the form cylinder is achieved in that a member rotatable around a central axis perpendicular to the pivot shaft of the doctor blade holder carries a pair of stud journals, the rotatable part of which carries the doctor blade holder and the central axis of which also extends in the center of the doctor blade holder at an angle perpendicular the central axis of the rotatable member. With this doctor blade apparatus, the doctor blade can be urged against the form cylinder with a good degree of parallelism because of the centered mounting of the doctor blade, allowing swing and bob movements. This known doctor blade apparatus is however not wholly satisfactory, since good abutting of the doctor blade against the surface of the form cylinder is obtained only when the blade is relatively short. This is because with longer blades and corresponding doctor blade holders, vibrations occur in the end regions thereof because of the central pivoting, said vibrations causing bad quality blading in the end regions of the form cylinder.
In another doctor blade apparatus disclosed in DE-AS 12 28 276, doctor blade holders are supported by a twistable doctor blade holder shaft having a rotating arm, said doctor blade holder shaft being journalled in pivot arms rotatable independently of each other, wherein an elastic force effective in the direction towards the form cylinder can be exerted on the pivot arm and on the rotating arm. This doctor blade apparatus also only works in a satisfactory manner if the doctor blade holder shaft supporting the lever-like doctor blade holders is not too long, since otherwise the doctor blade holder shaft can bend in its central area by an excessive amount, with the consequence that doctoring is imperfect in the center area of the form cylinder.